1. Field of the Invention
The present invention relates to a Non-Repeatable Run-Out (referred to as NRR hereafter) measuring instrument for a motor and it particularly relates to an NRR measuring instrument suitable for a thin-type HDD (Hard Disc Drive) employing a sensorless motor as a spindle motor.
2. Description of the Related Art
Presently, a thin-type HDD having a spindle motor is widely used for driving or rotating a disc on which information data are recorded and/or reproduced by using a magnetic head.
An NRR (Non-Repeatable Run-Out) is a significant criterion of a motor, particularly of a spindle motor of the HDD, to know how irregularly the motor rotates the disc of which a locus of its run-out is different at every turn i.e. non-cyclic, or how regularly the motor rotates the disc with a repeatable run-out locus.
As well known, in the HDD, information signals are recorded along a predetermined track on the disc by using the magnetic head, and the information signals recorded on the track are reproduced with the same magnetic head along the predetermined track. At that time, if a run-out locus of the disc at the reproduction is greatly different from that of the disc at the recording, i,e., the NRR of the motor is large, it is impossible for the magnetic head to follow the run-out locus and to reproduce the information signal from the recorded track correctly.
Thus, as a recent trend, the NRR for the spindle motor employed in the HDD is required to be below 0.3 .mu.m.
FIGS. 1 (A) through (C) are explanatory figures showing NRR measuring method of a prior art.
Referring to FIG. 1(A), a numeral 10 designates a 3-phase brushless motor (electrically commutated motor) having a rotor 10a exposed to an ambient outside, which motor 10 is an object of the NRR measuring, 12 an index mounted on a peripheral portion of the rotor 10a, 13 a driving circuit for driving the 3-phase brushless motor 10, 14 a noncontact displacement measuring instrument for measuring displacement or run-out of the rotor 10a without contacting the rotor 10a, 16 an index-sensor for sensing the index 12 and generating an index signal, and 18 an oscilloscope for displaying a displacement signal thereon generated from the noncontact displacement measuring instrument 14.
In this prior art, the displacement signal from the noncontact displacement measuring instrument 14 is observed on the oscilloscope 18 which is triggered by the index signal from the index-sensor 16, so that the displacement signal is repeatedly displayed on the oscilloscope 18 for every trigger cycle determined by the index signal, and the largest amplitude difference in displayed waveforms of the displacement signal caused at a time tx is defined as the NRR of the motor 10 as illustrated in FIG. 1 (A).
However, in the above mentioned method, if there is a large speed variation among respective revolutions of the motor 10, it is impossible to obtain the correct NRR of the motor 10.
For instance, if both the rotational speed variation and the NRR of the motor 10 is very small, then a simple wave form as shown in FIG. 1 (B) will be observed, however, if the rotational speed variation of the motor 10 (jitter) is very large, even if the NRR is actually very small, a complex waveform will be observed as shown in FIG. 1 (C) since a peak position of the waveform shifts between a time t.sub.1 and a time t.sub.2 due to the variation of the rotational speed. Accordingly, a large amount of the NRR will be seemingly observed on the oscilloscope 18 even when the actual NRR of the motor 10 is actually very small.
As an improvement of this NRR measuring method, there is another NRR measuring method of the prior art.
FIG. 2 is a schematic diagram showing the other NRR measuring method of the prior art.
Referring to FIG. 2, the object motor 10 to be measured is provided with an encoder disc 20 mounted on a spindle. In measuring the NRR, the revolution of the encoder 20 is detected by an encoder head 22, and encoder pulses from the encoder head 22 are inputted to a CPU (Central Processing port) 24 through an I/O (Input/Output Unit) 26.
On the other hand, displacement signal from the noncontact displacement measuring instrument 14 is converted into a digital signal by an A/D converter 28, and the digitized signal is inputted to the CPU 24 through the I/O 26. As a result, triggered by the encoder signal, sampled data are obtained from the digitized displacement signal.
TABLE 1 ______________________________________ trigger (n) .fwdarw. rev. (m).dwnarw. 1 2 3 ... 8 ______________________________________ 1 d(1,1) d(1,2) d(1,3) d(1,8) 2 d(2,1) d(2,2) d(2,3) d(2,8) 3 d(3,1) d(3,2) d(3,3) d(3,8) 4 d(4,1) d(4,2) d(4,3) d(4,8) . . . . . d(m,n) . . . . 64 d(64,1) d(64,2) d(64,3) d(64,8) ______________________________________
In this prior art, since eight encoder pulses are obtained per revolution of the motor 10, data sampling is performed eight times per revolution of the motor 10. Further, data sampling is conducted performed for 64 revolutions of the motor, as a result, displacement data d(m,n) (wherein m=1.about.64, n=1.about.8) are obtained as shown in TABLE 1.
In the CPU 24, a calculation is carried out to obtain the NRR of the motor 10 on the displacement data d(m,n) according to an expression (1) below. EQU NRR=MAX{n=1.about.8}[MAX{m=1.about.64}d(m,n)-MIN{m=1.about.64}d(m,n)](1)
In the expression (1), MAX{m=1.about.64}d(m,n) signifies the maximum value of the data d(m,n) among m=1.about.64 in the same column n, on the other hand MIN{m=1.about.64}d(m,n) signifies the minimum value of the data d(m,n) among m=1.about.64 in the same column n. In other words, the former and the latter expression respectively signify the maximum and the minimum data in every column containing 64 data in TABLE 1. Then, 8 remainders are obtained by subtracting the minimum values of data from the maximum values of data taken from respective columns. Thus, the NRR of the motor 10 is obtained as the maximum remainder among the 8 remainders.
However, this NRR measuring method requires installation of the encoder 20 every object motor 10 to be measured, which is disadvantageous in mass-production. Further, the installation of the encoder to a motor, particularly a thin motor, poses a problem that it is difficult to detect the displacement of peripheral portion of the rotor 10a because an installation of the noncontact displacement measuring instrument 14 is limited by the encoder 20.